1. Field of the Invention
The present invention relates to a signal processing apparatus and method for classifying an image into a plurality of groups according to the magnitude of signal values of the image, a noise reduction apparatus and method for performing noise reduction processing, and a computer program product for causing a computer to perform the signal processing method and noise reduction method.
2. Description of the Related Art
Pixel density and sensitivity of CCD's used in digital cameras and the like are ever increasing. The increase in the sensitivity of CCD's, however, has given rise to a problem of noise included in images obtained by photographing. Consequently, various methods have been proposed for reducing noise included in images. For example, a noise reduction method in which an amount of noise in each pixel of an image is estimated, as well as imaging conditions, then the estimated mount of noise is corrected according to the imaging conditions, and reducing the noise in the image based on the corrected amount of noise is proposed as described, for example, in U.S. Patent Application Publication No. 20050099515. Another method is also proposed as described, for example, in U.S. Patent Application Publication No. 20060039039, in which CCD-RAW data outputted from a CCD are separated into each of R, G, B color components, and noise reduction processing is performed while maintaining the correlation between each color component.
The method described in U.S. Patent Application Publication No. 20050099515 reduces the noise only by smoothing processing, though noise amount is estimated with respect to each pixel, so that the method could not reduce the noise appropriately when the amount of noise is large. In the mean time, the method described in U.S. Patent Application Publication No. 20060039039 could cause an image to be blurred when the photographing sensitivity is increased and hence the noise amounts to a large amount, since in such a case, distinction between the noise and signals of edges and the like is difficult or the correlation between each color component becomes small.